Method for producing lactams using oligophosphate catalysts

ABSTRACT

A process for preparing lactams by hydrolytic cyclization of aminonitriles in the gas phase in the presence of a metal phosphate catalyst comprises using a catalyst comprising one or more oligophosphates of the general formula (I) 
     
       
         M (PO a ) b   (I) 
       
     
     where M is a metal of group 3 or 4 of the periodic table, including the lanthanides, a is &gt;2.5 and &lt;4.0, and b is such that electrical neutrality is ensured, or a mixture of one or more oligophosphates of the general formula (I) with one or more further salts of a metal of group 3 or 4 of the periodic table, including the lanthanides, with an inorganic acid.

Lactams are versatile compounds. For instance, N-methylbutyrolactam(N-methylpyrrolidone) is a widely used solvent and e-caprolactam is animportant monomer for the manufacture of polyamide fibers.

Lactams can be prepared by hydrolytic cyclization of aminonitriles inthe gas phase. Catalysts having-dehydrating properties, such as aluminumoxide, silica gel or borophosphoric acid, are used.

EP-A 0 659 741 describes the preparation of lactams from aminonitrilesand water by hydrolytic cyclization in the gas phase using metalorthophosphates, especially aliminum, zirconium, niobium and lanthanumorthophosphates, as catalysts. The catalysts may additionally beimpregnated with basic alkali or alkaline earth metal compounds,preferably of cesium rubidium and potassium.

However, the selectivity of the prior art catalysts still leavessomething to be desired. The formation of by-products makes it difficultto isolate the lactams and may lead to poisoning of the catalysts used.

It is an object of the present invention to provide a catalyst forpreparing lactams by hydrolytic cyclization of aminonitriles that ishighly selective at high rates of conversion.

We have found that this object is achieved by a process for preparinglactams by hydrolytic cyclization of aminonitriles in the gas phase inthe presence of a metal phosphate catalyst, which comprises using acatalyst comprising one or more oligophosphates of the general formula(I)

M(PO_(a))_(b)  (I)

where M is a metal of group 3 or 4 of the periodic table, including thelanthanides, a is >2.5 and <4.0, and b is such that electricalneutrality is ensured, or a mixture of one or more oligophosphates ofthe general formula (I) with one or more further salts of a metal ofgroup 3 or 4 of the periodic table, including the lanthanides, with aninorganic acid.

The catalyst used may comprise one or more oligophosphates of thegeneral formula (I). Said formula (I) must be understood as grossstoichiometric formula and not as the molecular formula of actualexisting compounds. Oligophosphates for the purposes of the presentinvention are phosphates which are formally derived from acids which areobtainable by condensation of orthophosphoric acid with elimination ofwater. The condensation of orthophosphoric acid H₃PO₄ withintermolecular elimination of water yields chain-like oligophosphoricacids H_(n+2)P_(n)O_(3n+1) (tri-, tetra-, pentaphosphoric acid etc.;n=3, 4, 5, etc.) or (for large n) polymeric polyphosphoric acids.Triphosphoric and higher acids may also undergo an intramolecularcondensation to form ring-shaped metaphosphoric acids H_(n)P_(n)O_(3n)(tri-, tetrametaphosphoric acid etc.; n=3, 4, etc.), and not only achain-extending but a chain-branching condensation with the formation ofbranched ultraphosphoric acids (e.g., isotetraphosphoric acid H₆P₄O₁₃).The formal end product of the condensation is polymeric phosphoruspentaoxide P₂O₅. For the oligophosphates of the general formula (I)derived from these acids, a is between the corresponding value forpolymeric phosphorus pentaoxide (2.5) and that of orthophosphate (4.0).That is, 2.5 <a<4.0. a is preferably from 2.6 to 3.5, particularlypreferably from 3 to 3.5. In particular, a=3.

The choice of b is such as to ensure electrical neutrality. If thephosphorus in the oligophosphates is exclusively pentavalent phosphorus,b is especially (2a−5)/z, where z is the number of charges on the Mcations.

M is a metal of group 3 or 4 (=transition group III or IV, respectively)of the periodic table, including the lanthanides, i.e., Sc, Y, Ti, Zr,Hf, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu,preferably a lanthanide, particularly preferably La or Ce, veryparticularly preferably La. The oligophosphates used according to thepresent invention may contain one or more species of metals M,preferably they contain just a single species of metals M.

The catalyst used may be a specific oligophosphate, preferablytrimetaphosphate, or a mixture of a plurality of differentoligophosphates of the general formula (I).

A very particularly preferred catalyst is trimeta-phosphate, especiallylanthanum trimetaphosphate (LaP₃O₉)

The catalysts used may further be mixtures of one or more of theaforementioned oligophosphates of the general formula (I) with one ormore further salts of the aforementioned metals of group 3 or 4 of theperiodic table, including the lanthanides, with inorganic acids. Themetals present in the oligophosphates and in the further salts may beidentical or different; they are preferably identical. Preferred furthersalts are the orthophosphates, sulfates, carbonates, silicates,arsenites, arsenates, antimonites, antimonates and nitrates,particularly preferably the orthophosphates of the metals mentioned.

The oligophosphates of the general formula (I) may be used alone ormixed with the further salts. In general, the ratio of further salts tooligophosphates is up to 50:1, preferably up to 10:1, particularlypreferably up to 5:1, very particularly preferably from 0.1:1 to 5:1,and especially from 1:1 to 5:1.

The catalyst is further particularly preferably a mixture oftrimetaphosphate and orthophosphate, especially with lanthanum as metalM.

The oligophosphates of the general formula (I) and the further salts mayeach contain up to 5 molecules of water per unit formula.

The catalyst is generally prepared from the nitrate, nitrite, carbonate,formate, acetate, oxalate or some other salt of an organic acid, butpreferably from the nitrate of metal M and ammonium phosphate aspreferred precursors. These components are intimately mixed with eachother in the desired molar ratio as fine powders. The chosen molar ratioof phosphorus:metal (=b) will be present in the product (theoligophosphate or the mixture of different oligophosphates) after thereaction has taken place. After mixing, the precursors are slowlyheated, for example in an open crucible, at temperatures from generally140 to 200° C., preferably from 150 to 180° C., for generally from 2 to48 hours, preferably from 8 to 36 hours, to decompose the precursors.This is followed by gradual heating to 250-900° C., preferably 400-650°C., for 1-8 days, preferably 2-5 days, to complete the conversion intothe oligophosphate. This method makes it possible to obtain anyoligophosphates having a phosphorus:metal ratio=3 as mixtures, but also,in some instances, in pure form. Trimetaphosphate, for example, is thusobtainable in pure form.

The present invention further provides a process for preparing acatalyst, which comprises the steps of:

a) preparing a mixture of ammonium dihydrogen-phosphate and the nitrateof said metal M in the desired molar ratio;

b) heating this mixture in stages to decompose the precursors and formthe metal oligophosphate in a solid state reaction.

Metaphosphates are further obtainable by precipitating M asdihydropyrophosphate from a solution of a salt of M with pyrophosphoricacid H₄P₂O₇ and calcining the resulting precipitate to form themetaphosphate.

The present invention further provides a process for preparing acatalyst, which comprises the steps of:

a) preparing a solution comprising a salt of said metal M;

b) precipitating said metal M from this solution as dihydropyrophosphateand removing the precipitate comprising the metal dihydropyrophosphate;

c) optionally washing and drying said precipitate;

d) calcining said precipitate.

The precipitating of M as dihydropyrophosphate is preferably carried outat a pH of generally 0.5-4, preferably 0.8-2. To effect precipitation, abase may be added to a solution comprising the salt of metal M andpyrophosphoric acid to establish a certain pH. Preferred bases areammonia, alkali metal hydroxides, primary, secondary and tertiaryamines, particularly preferably ammonia. It is further possible to add asolution comprising the salt of metal M to an aqueous alkaline solutionof pyrophosphoric acid. Suitable salts are water-soluble salts of metalM, preferably nitrates.

The concentration of the metal salt solution is generally from 0.1-1.5mol/l, preferably 0.8-1.1, and that of the pyrophosphoric acid isgenerally 0.1-5, preferably 2-4, mol/l.

The precipitating of M as pyrophosphate may be carried out in the coldor in the heat. To prepare catalysts which consist essentially ofoligophosphates of the general formula (I) and do not containsignificant portions of orthophosphates, it is preferred to carry outthe precipitation in the cold, particularly preferably by cooling withice. To prepare catalysts which contain orthophosphates as furthersalts, the precipitation is preferably carried out in the heat, in whichcase the temperature is generally 30-100° C., preferably 60-90° C.

The pyrophosphate-containing precipitate is separated off, optionallywashed and dried and subsequently calcined. The drying generally takesplace at from 60 to 180° C., preferably at from 100 to 150° C., and thecalcining generally at from 300 to 900° C., preferably at from 500 to700° C., for generally 0.5-10 h, preferably 2-4 h.

Mixtures of oligophosphates of the general formula (I) and one or morefurther salts may be obtained by one of the following methods:

by solid state reaction of a nitrate of metal M, ammoniumdihydrogenphbsphate and an ammonium salt of the inorganic acid fromwhich the further salts were derived;

by coprecipitation of dihydropyrophosphates of metal M and further saltsand subsequent calcination;

by evaporating solutions comprising phosphoric acid or oligophosphoricacids, optionally other inorganic acids and also the metal M, whichoriginates from the corresponding oxide in the desired molar ratio, andoptionally subsequent calcination.

The catalyst materials may be used in any desired form, for example aspowders, as spall or as molded shapes. Examples of molded shapes for thecatalyst materials are extrudates or spheres. A binder may be added toproduce the molded shapes, for example Aerosil, potato starch orcellulose ether (e.g., Walocel®). The catalyst materials may further beapplied to a support, such as argillaceous earth, silica gel, carbon,silicon carbide or silicon nitride.

The catalyst is preferably used in the form of spall or molded shapes.The catalyst bed may have mixed into it additional,selectivity-enhancing components, in an amount of up to 70% by volume.Examples are silicon dioxide, silicon nitrite and silicon carbide,preferably silicon dioxide, particularly preferably quartz. Usefulaminonitriles for the process of the present invention are aliphaticaminonitriles having at least two, preferably from 3 to 20, atoms in thechain between the amino group and the nitrile group. In general, theseatoms are carbon atoms, but it is also possible for the chain to containone or more, but preferably not more than 3, boron, nitrogen,phosphorus, oxygen and/or sulfur atoms in nonadjacent, but otherwisediscretionary position. The amino group may be monosubstituted by astraight-chain or branched alkyl group having up to 20 carbon atoms, forexample by methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl ortert-butyl. The aminonitriles used in the process of the presentinvention are preferably aminonitriles having 3, 4, 5 or 6 carbon atomsin the chain between the amino and the nitrile group without heteroatoms, particularly preferably with primary amino groups, such as4-aminobutyronitrile, 5-amino-rvaleronitrile, 6-aminocapronitrile and7-aminoenantho-nitrile, very particularly preferably6-aminocapro-nitrile.

The reductive cyclization can be carried out over a moving catalyst bedor over a stationary catalyst bed. The reaction is preferably carriedout over a stationary catalyst bed (fixed bed). The fixed bed may, forexample, take the form of a single dumped bed or be subdivided into aplurality of trays. The fixed bed may also be disposed in one or morereaction spaces, for example in a tube bundle reactor. The molar ratioof water to aminonitrile is generally within the range from 1 to 50,preferably within the range from 1 to 15. The reaction temperature isgenerally within the range from 200° to 550° C., preferably within therange from 300° to 400° C. Temperatures below 200° C. slow thevaporization of the aminonitrile and make it difficult to achieve highconversions. Temperatures above 550° C. give rise to increased formationof by-products and decomposition products.

The reaction is generally carried out at a pressure from 0.01 to 10 bar,preferably at atmospheric pressure.

The reaction may be carried out in the presence of an inert gas, forexample argon or nitrogen, in which case the inert gas may be present inan excess of up to 100-fold based on the aminonitrile.

The reactor effluent, as well as the lactam product, generally comprisesunconverted aminonitrile and water and also ammonia or amines and minoramounts of byproducts such as aminocarboxamides. The lactam may berecovered from the reaction effluent by customary separation processessuch as distillation, extraction or crystallization.

Catalyst space velocity is typically within the range from 50 to 2000 g,preferably within the range from 500 to 2000 g, of aminonitrile perliter of catalyst per hour. Conversions based on aminonitrile are withinthe range from 70 to 99.9%. The selectivity of lactam formation isgenerally above 85%, preferably above 90%, particularly preferably above93%, based on amino nitrile used. Selectivities above 95% are possible.These selectivity values are achieved even after catalyst on-streamtimes of several 100 hours.

The Examples which follow illustrate the invention.

Catalyst Preparation Catalyst 1

Finely triturated powders of (NH₄)H₂PO₄ and La(NO₃)₃.6H₂O are intimatelymixed with each other as precursors in a molar ratio of 3:1 in aporcelain crucible. To decompose the precursors, the mixture ismaintained at 150° C. for 24 h and then at 180° C. for 12 h andsubsequently slowly heated to 600° C. After four days, the material iscooled down to obtain a solid melt of LaP₃O₉, which is readilycomminutable and processible into spall from 0.1 to 0.5 mm in particlesize.

Catalyst 2

Solutions are prepared of 0.5 mol of La(NO₃)₃.6H₂O in 500 ml of waterand of 1.5 mol of pyrophosphoric acid in 500 ml of water. The firstsolution is added dropwise to the second solution with stirring.Thereafter 3 mol of an aqueous concentrated NH₃ solution diluted withwater in the ratio of 1:1 are then added dropwise while cooling withice, and a precipitate is formed. This precipitate is separated off,washed with cold NH₃ solution and then dried at 150° C. for 18 h. Thematerial thus obtained is comminuted and processed into spall from 0.1to 0.5 mm in particle size. Thereafter the material is decomposed to themetaphosphate at 380° C. for 9.5 h and then at 550° C. for 2 h.

Catalyst 3

A solution is prepared of 0.8 mol of La(NO₃)₃.6H₂O in 736 ml of waterand a solution of 0.92 mol of pyrophosphoric acid in 800 ml of water.The pyrophosphoric acid is adjusted to pH 10 with concentrated NH₃solution, and then the metal salt solution is slowly added dropwise. Theresulting precipitate is stirred at 80° C. for 1.5 h. The precipitate iscentrifuged off, slurried up twice with ammoniacal water of pH 10.0 andagain centrifuged off. The material thus obtained is dried at 110° C.for 12 h and thereafter processed into spall from 0.1 to 0.5 mm inparticle size. The material is then calcined at 700° C. for 4 h to givea mixture of meta- and orthophosphate.

Catalyst C (Comparative Example)

The directions of EP-A 0 659 741 are followed to prepare a catalystconsisting of pure lanthanum orthophosphate. To this end, a solution of1.0 mol of La(NO₃)₃.6H₂O in 3000 ml of water and a solution of 2.0 molof (NH₄)₂HPO₄ in 1500 ml of water are prepared. The second solution isslowly added dropwise to the first at room temperature with stirring,and a precipitate is formed. Thereafter the pH of the suspension isadjusted to 6.0 with aqueous NH₃ solution. After stirring for 30minutes, the precipitate is washed with 24 l of water on a suctionfilter and thereafter dried at 120° C. for 12 h. The material obtainedis efficiently comminutable and processed into spall from 0.1 to 0.5 mmin particle size. The spall is finally calcined at 500° C. for 4 h.According to its XRD spectrum, the calcination product consists of purelanthanum orthophosphate.

Cyclization Experiment

The above-described catalysts are tested in an electrically heatedtubular reactor 30 mm in internal diameter, packed (starting at thebottom) with 20 ml of quartz spall, then 20 ml of catalyst as spall <0.1mm and then 50 ml of quartz spall as vaporizer zone. After packing withcatalyst, the reactor is in accordance with EP-A 0 659 741 heated to400° C. in an air stream and then cooled down under nitrogen to thereaction temperature.

The reactor is operated in downflow mode. 6-Amino-capronitrile ischarged as 50% strength by weight aqueous solution at 750 g per 1 ofcatalyst per hour. The reaction takes place at 360° C. at atmosphericpressure with the addition of 10 l/h of nitrogen as carrier gas. Theconversion of 6-aminocapronitrile (ACN) and the selectivity forcaprolactam (CPL) are determined by means of gas chromatography using aninternal standard and via the mass balance. Samples are accumulated overseveral hours for exact quantitative measurement of selectivity andconversion. The results are summarized in Table 1.

TABLE 1 Time of Catalyst CPL selectivity ACN conversion measurement 193.5% 99.2%  53 h 91.7% 99.2% 124 h 2 92.4% 99.3% 165 h 92.9% 99.3% 555h 3 98.4% 99.6% 276 h 97.0% 99.6% 494 h C 85.8% 99.7%  70 h 88.5% 99.3%166 h

The measurements were carried out after the reaction had proceeded understable conditions for at least two days. The analysis of catalyst Cafter 166 h revealed that it was still pure lanthanum orthophosphate.

The Examples show that the oligophosphate-containing catalysts 1 to 3used according to the invention give higher conversions and caprolactamselectivities than a pure lanthanum orthophosphate (catalyst C).

We claim:
 1. A process for preparing caprolactam which comprises:hydrolytically cyclizing 6-aminocapronitrile in the gas phase in thepresence of a metal phosphite catalyst in which the catalyst is one ormore oligophosphates of the formula (I) (M(PO_(a))_(b)  (I) where M isLa, a is from 2.6 to 3.5 and b is such that electrical neutrality isensured, or a mixture of one or more oligophosphates of the formula (I)with one or more further salts of a metal of group 3 or 4 of theperiodic table, including the lanthanides, with an inorganic acid. 2.The process of claim 1, wherein said further salts are selected from thegroup consisting of orthophosphate, sulfate, carbonate, silicate,arsenite, arsenate, antimonite, antimonate and nitrate.
 3. The processof claim 1, wherein said oligophosphate is trimetaphosphate.
 4. Theprocess of claim 1, wherein the molar ratio of said further salts tosaid oligophosphates of the formula (I) is within the range from 1 to 5.5. The process of claim 1, wherein said further salt is orthophosphate.6. A process for preparing a catalyst as described in claim 1, whichcomprises the steps of: a) preparing a solution comprising a salt ofsaid metal M; b) precipitating said metal M from this solution asdihydropyrophosphate and removing the precipitate comprising the metaldihydropyro-phosphate; c) optionally washing and drying saidprecipitate; d) calcining said precipitate.
 7. A process for preparing acatalyst as described in claim 1, which comprises the steps of: a)preparing a mixture of ammonium dihydrogen-phosphate and the nitrate ofsaid metal M in the desired molar ratio; b) heating this mixture instages to decompose the precursors and form the metal oligophosphate ina solid state reaction.